1. Field of the Invention
The present invention relates to an electrostatically stimulated fluid filter, and more specifically to an electrostatically stimulated air filter having an upstream control grid, an ionization grid, a downstream filter having a conductive backing, and ground potential grid.
2. Description of Related Art
Conventional electrostatically stimulated filtration (i.e., "ESF") devices, with or without pre-chargers/ionizers, use a dielectric filter media interposed between a high voltage and ground potential flat porous metal grids or screen electrodes. See Jaisinghani, R. A., Hamado, T. A., Hawley, C. W., Electrically Stimulated Filter Method and Apparatus, U.S. Pat. No. 4,853,005, filed 1 Aug. 1989 (hereinafter Jaisinghani '005). Typically, the filter media is in pleated form to increase surface area and thereby facilitate air flow and dust collection. The pleated form, however, results in a relatively high gap between electrodes. This gap requires the application of high voltage, typically 16-28 kilo-Volts, kV, in order to achieve the required high applied field strengths (typically 1.5 kV/cm to 2.5 kV/cm). See Jaisinghani, R. A. and T. A. Hamade; Effect of Relative Humidity on Electrically Stimulated Filter Performance, J. of APCA, Vol 37, 823-828, 1987 (hereinafter Jaisinghani and Hamade); R. A. Jaisinghani and N. J. Bugli; Performance Characteristics of a Two Electrode Ionizing Electrically Stimulated Filter, Proceeding Symposium on Contamination Control and Clean Room Tech., 19th Ann meet, Fine Particle Society, Santa Clara, Calif., July 1988; R. A. Jaisinghani and N. J. Bugli; Advantages of Electrically Stimulated Filtration over Conventional Filtration, Fluid/Particle Sep. J., Vol 1, No 2, 1988. In contrast, in rare cases fiat depth filter mats are used.
The requirement of high electrical potentials and the application of high voltage directly to the filter have several drawbacks. First, the materials used in the construction of the filter must have high dielectric strength and high resistance to arc tracking, increasing the cost of such filters. Second, the cost of the high voltage power supply is expensive since its cost is dependent upon the output voltage, among other variables such as power. Third, special attention is needed in the design of filter housings since large insulating air gaps and insulators are required between the high and low potential components. Fourth, components at very high voltage can be a safety hazard since under certain conditions sparks occur which can ignite the filter components. Fifth, as the high voltage components and inside ground potential surfaces of the filter housing become contaminated, conductive tracks result which draw power from the high voltage supply, and in some cases results in short circuits and sparks (n.b., many common aerosols, such as cigarette smoke, are fairly conductive, especially when carbonized within the high fields). All these factors necessitate cost additions which increase the cost of ESF technology and thereby reduce its application.
The requirement of the very high voltage can be partially overcome by using thin glass paper filter media along with corrugated aluminum spacers in between pleats of this filter media. See Masuda, S., High Efficiency Electrostatic Filter Device, U.S. Pat. 4,509,958, issued on 9 Apr. 1985. These corrugated aluminum spacers arc alternately connected to high and low voltage output of a power supply. This configuration significantly reduces need for high voltage. Since the corrugated aluminum spacers have some depth, the main field strength is approximately equal to the distance between the centerlines of corrugated electrodes which is approximately equal to the pleat spacing. One limitation or drawback of this configuration is that the corrugated aluminum spacers tend to tear the delicate fiber glass paper filter media. This possibility of tearing results in quality control problems that negate the advantages of the lower voltages. Another limitation of this configuration is that the high voltage must be applied directly to each the filter element. The application of the high voltage is typically accomplished with flat metal grid electrodes contacting each corresponding spacer. This results in additional cost, since the manufacturing process must ensure that each and every spacer is in contact to the corresponding grid electrode--no easy task since an clement may have over a hundred spacers. Further, as the filter material becomes relatively conductive due to contamination the power requirement increases exponentially and additionally there is a danger of igniting the filter material. This effect also occurs in a high relative humidity atmosphere.
Newer designs have eliminated the need for direct application of a high voltage to the filter by incorporating the filter within the ionizer using a single ground electrode. See Jaisinghani, R. A. and N. J. Bugli; Single Field Ionizing Electrically Stimulated Filter, U.S. Pat. No. 4,940,470, issued on 10 Jul. 1990 (hereinafter Jaisinghani '470). The primary advantage of this design is that only one high voltage electrode is used simplifying the construction of the housing. On the other hand, this design has several drawbacks. First, since a dielectric material is between the high voltage ionizing wires and the ground plate, ionization is suppressed at moderate field strengths and extremely high voltages, at least 26 to 28 kV is required in order to achieve adequate charging of incoming particles. Second, the required high voltages create all the disadvantages associated with previously discussed conventional ESF technology. Third, as the filter becomes contaminated, the field strength in the gap between the filter and the ionizing wires increases causing an increased current and sparking towards the potentially combustible filter, necessitating a highly regulated power supply which is costly.